Universal joint transmissions are made to transmit rotation and torque from a power take-off to a driven shaft.
In the presence of torques beyond design threshold, due for example to the intervention of foreign bodies, some elements of the universal joint transmission system or of the mechanism arranged downstream might undergo structural failure and also risk compromising the safety of the operator.
For this reason, several solutions are known which ensure protection from peak loads of the entire universal joint transmission system and of the mechanism arranged downstream.
In one of the solutions of the known type, the external rotatable element, connected to the source of driving torque, transmits the motion to a driven hub by means of a plurality of pawls. On the head of the pawls there is a shaped profile which engages adapted recesses or slots, provided on the inner surface of the external rotatable element. The head of each pawl is pressed against the recesses by means of a shaped foot of the pawl, which is kept in contact with wedges capable of pushing radially outwardly the pawls by way of elastic means. Torque transmission occurs by way of the tangential force that develops between the profile defined by the recesses in the external rotatable element and the head of the pawls. As the value of the resisting moment of the driven hub increases, a radial thrust on the pawl is generated which overcomes the contrast force generated by the elastic means, disengaging the head of the pawl from the corresponding recess. The head of the pawl therefore slides on the inner surface of the external rotatable element and the value of the transmitted torque decreases suddenly and is then restored when the pawls reengage the recesses of the external rotatable element.
These solutions of the known type are not devoid of drawbacks, including the fact that, during the relative rotation between the external rotatable element and the driven hub, the pawls are subjected to impacts that are repeated cyclically for the entire time during which the transmission remains uncoupled.
Another drawback of these solutions of the known type resides in that, because of the impacts, the useful life of the pawls and of all the components directly or indirectly subjected to the impacts is greatly reduced.
Another drawback of these solutions of the known type resides in that they have an early wear of the coupling and/or uncoupling system, with consequent alteration of the geometries and of the surfaces assigned to the exchange of actions and reactions that compromise their functionality.